Thermal storage systems typically use inexpensive off-peak electric power, solar energy or waste heat to meet heating, cooling or hot water requirements. Ice and water have been the traditional storage media to store the thermal energy, but with inherent constraints. Ice storage is limited to the working temperature of around 0° C., which is too low for many applications such as air-conditioning, and is energy inefficient. Water, while convenient to harness, has limited heat storage capacity, as it relies on sensible heat in the temperature range of only 0 to 100° C., and the water storage tank is usually bulky.
Microencapsulated phase change materials (MPCM) offer the flexibility of a wide range of working temperatures when suspended in water to form an aqueous slurry in a thermal storage tank. Phase change materials contained within microcapsules may be frozen into solid states through refrigeration to effect cooling storage. MPCM contained within microcapsules may be melted into liquid state by solar energy or waste heat to effect heat storage. The phase change materials may be used to store thermal energy by cycling between solid and liquid phases. When a liquid material is solidified, heat is released, providing a heating effect, with the accompanying absorption and release of heat, which accomplish the heating or cooling effects.
Therefore, the thermal storage systems that incorporate MPCM may be used to store energy. This thermal storage can allow electricity usage to be shifted towards periods of the day with lower electricity costs. This redistribution of electricity usage can allow peak shaving, which may result in the reduction of the overall electricity capacity requirement. Such a system can also be used to store thermal energy available from natural evaporative cooling and solar heating.
However, MPCM slurries have been used in cooling applications only at low concentrations. One reason for this limitation is that the slurries present non-Newtonian behaviors when the particle volume fractions are higher than about 30 percent. A low MPCM particle concentration corresponds to lower heat storage capacity for a given volume of a storage tank. Moreover, the breakage of particles, which can result from the impact with the pump, may lead to higher pump energy consumption due to agglomeration of the material.
It is desirable to develop a new thermal storage system operating with an MPCM slurry having a particle concentration higher than 30 percent. It is also desirable that such a system has minimal breakage of the MPCM during its performance life.